1. Field of the Invention
The present invention relates to an antenna system for a cellular mobile telephone operating by radio channel between said mobile and a group of stations revolving in polar orbit or quasi polar orbit around the earth.
The invention also relates to a mobile telephone including such antenna system.
Within the framework of the diversification and extension of cellular mobile telephone networks, the number of base stations is continuously increasing in densely populated areas, which are on the way to being completely covered by the conventional networks of the main operators. The problem of coverage is nevertheless posed for other geographical areas. It is particularly important in desert areas or maritime regions.
The invention is more particularly adapted to new operators who propose to mitigate these flaws of conventional networks by proposing cellular systems that operate by radio channel between the mobile and a group of satellites revolving in polar orbit around the earth. The various projects differ in their state of progress, the precise frequency bands used of the order of 1 or 2 GHz, the numbers of satellites and their orbits, and in the status of the agency that provides the network. For these new networks and for this type of channels, mobile telephones are to be provided which also have new antennas and which antennas can no longer be reduced to a single strand, because the satellites (that is to say, the base stations) with which communication is effected, be this either in the calling mode or in the stand-by mode, may be several thousand kilometers apart in space.
2. Description of the Related Art
The concept and the choice of the antennas for such mobile telephones is particularly critical in the case of a network by satellite: the main reasons therefor are the distances between the base station and the mobile, the circular polarization required for the waves, the result of the gain/temperature ratio of noise on reception and the various positions the mobile may take up relative to the satellite as a function of the use of the former and the position of the latter. Moreover, the concept of the mechanical reference system does not exist in the mobile application: any definition of a polarization angle is illusive, because the user adjusts the telephone antenna to any angle that evolves when he moves and it would be impossible to impose a vertical or horizontal angle of polarization.
The receiving quality depends on the gain of the antenna, but also on the total of its radiation diagram that is to present low values in the noisy directions as regards radioelectricity, that is to say, the directions, in essence, towards the ground. The quality criterion currently used is furthermore the ratio between the antenna gain in the direction of the received waves and the total temperature of received noise, that is, G/T.
For the application to mobile telephony, the antenna is to optimize this criterion as much as possible during the communication phases, during which phases the user holds his telephone in an approximately upright position, without excessively degrading this criterion during the stand-by phases, in order to permit sufficient operation for making contact with a user who has put his mobile on a flat horizontal support. When in communication, the solid angle to be covered is 2 .PI. steradians along the main axis of the telephone, above the horizontal plane. Conversely, when the mobile is put down again, the interesting angle for receiving signals coming from a satellite close to the perigee is perpendicular to its main axis. that is, to the side of the telephone, which corresponds to the horizon if the mobile is put back in its normal (upright) position and the gain is thus to have a certain value in this direction.
An antenna in the form of a conical spiraled antenna with various strands is chosen as a circularly polarized antenna, intended for the implementation of the invention. Such an antenna is known per se from U.S. Pat. No. 4,656,485. In this case, it is an antenna that is fixed to the ground, designed for transmitting and receiving short waves at frequencies of the order of several MHz. It may function in two switching modes via a supply reversal of 2 of its 4 strands so as to obtain omnidirectional radiation diagrams with larger or smaller angles of elevation and, besides, with different frequencies depending on the mode of excitation.